Although some edible oils are used per se, by far the largest portion are hydrogenated, or hardened, prior to their end use. The reason for such hydrogenation is to increase the stability of the final product. For example, processed soybean oil is susceptible to oxidation resulting in deterioration of its organoleptic properties upon storage even at ambient temperature. Where the oil is to be used at higher temperatures, for example, as a frying oil, the adverse organoleptic consequences of oxidation become even more pronounced.
The commonly accepted origin of oxidative deterioration is the presence of highly unsaturated components, such as the triene moiety, linolenate, in soybean oil. Partial hydrogenation to remove most of this component leads to a marked increase in the oxidative stability of the resulting product, thereby facilitating storage and permitting unobjectionable use at higher temperatures. Ideally, one desires this hydrogenation to be highly specific, reducing only triene to the diene, linoleate, without reducing diene or monoene and without effecting cis to trans isomerization. In practice, this goal is unachievable.
The edible fats and oils which are the subject of this invention are triglycerides of fatty acids, some of which are saturated and some of which are unsaturated. In vegetable oils, the major saturated fatty acids are lauric (12:0), myristic (14:0), palmitic (16:0), stearic (18:0), arachidic (20:0), and behenic (22:0) acids. The notation, "18:0," for example, means an unbranched fatty acid containing 18 carbon atoms and 0 double bonds. The major unsaturated fatty acids of vegetable oils may be classified as monounsaturated, chief of which are oleic (18:1) and erucic (22:1) acids, and polyunsaturated, chief of which are the diene linoleic acid (18:2), and the triene linolenic acid (18:3). Unhardened vegetable fats and oils contain virtually exclusively cis unsaturated acids.
In the context of partial, light hydrogenation, the ultimate goal is the reduction of triene to diene without attendant trans acid formation or saturate formation. In practice, it is observed that partial reduction results in lowering both triene and diene and increasing the monoene, saturate, and trans levels. Because it is desired that the product of partial hydrogenation itself be a liquid oil relatively free of sediment or even cloudiness upon storage at, for example, 10.degree. C., the formation of saturated and trans acids in such hydrogenation is a vexing problem. Removal of these solids, whose relative amount is measured by the Solid Fat Index (SFI), is a relatively costly and inefficient process attended by large losses associated with the separation of gelatinous solids from a viscous liquid. It is known in the art that such solids are composed largely of triglycerides containing at least one saturated fatty acid moiety and/or trans monounsaturated fatty acid moiety with the predominant culprits having at least 18 carbon atoms. It is further known in the art that fatty acid analysis alone is an insensitive analytical tool, that is to say, two products of hydrogenation of, for example, soybean oil may be vastly different in their SFI while having virtually identical fatty acid analysis. This arises because the distribution of the saturated moieties in the triglyceride is important. The solubility in the soybean oil of disaturated triglycerides is much less than twice that of monosaturated triglycerides, and the solubility of monosaturated triglycerides may depend upon whether the other fatty acid moieties of the triglyceride are monounsaturated, diunsaturated, etc., and may also depend upon whether the saturated portion is at the one or two position of the triglyceride.
In the context of more extensive but still partial hydrogenation selectivity remains a goal. What is particularly desired is to reduce triene and diene levels without an accompanying substantial increase in saturates, i.e., conversion of triene and diene to monoene as exclusively as possible. Another frequent goal in partial hydrogenation also associated with selectivity is formation of a product with a steep SFI slope. By this is meant a product with a high SFI in the interval from 10.degree. C. to about 27.degree. C. decreasing rapidly in the interval from about 27.degree. C. to about 40.degree. C., with little or no solids at about 40.degree. C.
From the foregoing it should be clear that hydrogenation of edible fats and oils is largely an empirical process, whose analytical tools include SFI supported by fatty acid analysis. The difficulty of achieving desirable results, in the context of selectivity in Solid Fat Index, has largely limited such hydrogenation to a batch type process. Although the transition from a batch to a continuous process, especially of the fixed bed type, is conceptually facile, it will be recognized by the skilled worker that impediments have been substantial.
I have discovered that phosphorus-modified nickel catalysts display virtually unchanged activity but substantially increased selectivity relative to the unmodified nickel in the hydrogenation of fatty materials. Such catalysts appear to consist, at least in part, of nickel phosphide or related species. The latter has been shown to reduce 1,3-butadiene, a conjugated diene, (F. Nozaki and R. Adachi, J. Catalysis, 40, 166 (1975)) by exclusively 1,4-addition. Since the double bonds in the fatty materials of this invention are isolated double bonds, i.e., non-conjugated double bonds, it is surprising to find the nickel phosphide specie(s) of this invention participating in 1,2-reduction, a conclusion which follows from the unaltered, or sometimes increased, catalytic activity. It is also totally unexpected that the catalysts of this invention are so selective in reducing the isolated double bonds of triene and diene relative to monoene. As a consequence of my discovery I have developed a method of selectively hydrogenating fatty materials, which is my invention. An important advantage of my invention is that it provides a method of continuous hydrogenation of fatty materials with good selectivity, a goal long pursued but commercially unrealized.